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Frame Structures in LTE-TDDand LTE-FDD

Lauro7 Aug 2012 6:08 PM 0

So far, we have concentrated our efforts in describing LTE-FDD. However, as TD-LTEgets deployed in the USA, more blogs on this technology will be published.

In this occasion, we will describe the frame structure of both, LTE-FDD and LTE-TDD.

Let us describe the LTE-FDD structure first.

The duration of one LTE radio frame is 10 ms. One frame is divided into 10 subframesof 1 ms each, and each subframe is divided into two slots of 0.5 ms each. Each slot

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contains either six or seven OFDM symbols, depending on the Cyclic Prefix (CP) length.The useful symbol time is 1/15 kHz= 66.6 mircosec. Since normal CP is about 4.69microsec long, seven OFDM symbols can be placed in the 0.5-ms slot as each symboloccupies (66.6 + 4.69) = 71.29 microseconds. When extended CP (=16.67 microsec) isused the total OFDM symbol time is (66.6 + 16.67) = 83.27 microseconds. Six OFDM

symbols can then be placed in the 0.5-ms slot. Frames are useful to send systeminformation. Subframes facilitate resource allocation and slots are useful forsynchronization. Frequency hopping is possible at the subframe and slot levels.

In LTE, radio resources are allocated in units of Physical Resource Blocks (PRBs).Each PRB contains 12 subcarriers and one slot. If the normal Cyclic Prefix is used, aPRB will contain 12 subcarriers over seven symbols. If the extended CP is used, thePRB contains only six symbols. The UE is specified allocation for the first slot of asubframe. There is implicit allocation for the second slot of the subframe. For example,if the eNB specifies one RB as the resource allocation for the UE, the UE actually usestwo RBs, one RB in each of the two slots of a subframe. When frequency hopping isturned on, the actual PRBs that carry the UE data can be different in the two slots. In a10 MHz spectrum bandwidth, there are 600 usable subcarriers and 50 PRBs.

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Frame structure Type 2 is applicable to TDD is as shown in the figure. Each radio frameof 10 ms in length consists of two half-frames of 5 ms in length. Each half-frameconsists of eight slots of the length Ts=5 ms and three special fields DwPTS, GP, andUpPTS of 1 ms in length.

Different configurations, numbered zero to six, are defined in the standard for thesubframe number allocated for the uplink and downlink transmission. Subframe 1 in allconfigurations and subframe 6 in configurations 0, 1, 2 and 6 consist of DwPTS, GP andUpPTS. All other subframes are defined as two slots.

Switch-point periodicities of 5 ms and 10 ms are supported. The standard defines thetable for the uplink and downlink allocations for switch-point periodicity. In the case of a5-ms switch-point periodicity, UpPTS and subframes 2 and 7 are reserved for uplinktransmission.

In the case of a 10-ms switch-point periodicity, UpPTS and subframe 2 are reserved foruplink transmission and subframes 7 to 9 are reserved for downlink transmission.

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Subframe 0 and 5 are always for the DL. The subframe following the special SF isalways for the UL. The DwPTS field carries synchronization and user data as well asthe downlink control channel for transmitting scheduling and control information. TheUpPTS field is used for transmitting the PRACH and the Sounding Reference Signal(SRS).